Maternal serum screening in the second trimester indicates increased risk of Down's syndrome

When a woman has previously had a pregnancy affected by a chromosomal or genetic disorder.

When arelative has had a baby with Down's syndrome

If an ultrasound examination has detected features or abnormalities indicating an increased risk of a chromosome abnormality.

If woman requests it because she is concerned that her baby may have a chromosome abnormality.

Amniocentesis is occasionally required in the investigation and management of other conditions in pregnancy such as rhesus diseases, infection or the need to deliver the baby prematurely. In these circumstances, the amniotic fluid is analysed for substances or bacteria or virus in the fluid itself and not specifically to look at the baby's chromosomes.

Why the test is performed

The test can detect chromosomal disorders such as Down's syndrome, structural defects such as spina bifida (open spine, where the vertebrae fail to close), anencephaly (a condition in which the brain is incomplete or missing), and many rare, inherited metabolic disorders.

Later on in a pregnancy, the test may be used to identify suspected problems such as Rh incompatibility or infection or lung maturity.

The uterus is examined by Ultrasound to asess

Pregnancy is far enough to perform the test

There is only one fetus in the uterus

Best area to insert the needle without injuring the fetus

Best area to insert the needle without injuring the placenta

Ultrasound can also detect some serious birth defects

Amniocentesis is performed by placing a needle into the uterus through the lower abdomen. The needle is the same thickness as those used to draw blood from your arm or to give you an injection. A small amount of local anesthesia may be injected into your skin to numb the area before the amniocentesis needle is inserted.

A small amount 20 mls of amniotic fluid is removed from the uterus for testing.

This fluid is replaced very rapidly by the pregnancy.

Many women are concerned about how painful the test procedure might be. In fact, amniocentesis is usually not any more painful than having an injection in any other area of the body.

The procedure usually takes 5 to 10 minutes

After the procedure, you should rest for the day.

You may feel slight cramping.

Bleeding and leaking of fluid from the vagina is not normal and you should contact the hospital if this happens.

Rheusus negative mums

If your blood group is rhesus-negative (one in eight women have this blood group), you will be given an injection of human anti-D immunoglobulin into a muscle of your arm or leg. Anti-D immunoglobulin is a substance administered to rhesus-negative women when they have been potentially exposed to rhesus-positive blood cells, for example after giving birth, a threatened miscarriage, or amniocentesis. Its purpose is to prevent the mother developing antibodies against the foetal blood cells, which can cause severe jaundice or even foetal death. The use of Anti-D immunoglobulin has been one of the success stories of the last 20 years.

How long will it take to know the result?

A preliminary result may be available within 48 hours and the final result will be known in two to three weeks.

Is amniocentesis safe for the mother and baby?

Amniocentesis is not risk-free and carries a slight risk of causing a miscarriage. Between 1 in every 100 and 1 in every 200 amniocentesis procedures will cause a miscarriage, usually within three weeks of the procedure.

Before deciding upon screening and diagnostic tests you should discuss any concerns you may have with your obstetrician. Having information about the baby's chromosomes has to be balanced against the small but real risk of losing the pregnancy.

How reliable is the result from an amniocentesis?

Amniocentesis has a certainty of 99.9 per cent in identifying chromosome abnormalities.

What happens if the test shows abnormalities?

There is no treatment for chromosome abnormalities. If a diagnosis is made before birth, and depending on the nature of the abnormality that has been detected, you will have options and termination of pregnancy is one of them.

The decision whether or not to continue with the pregnancy always rests with you and your partner and some parents may decide to continue.

Before an amniocentesis test is carried out, it is therefore important for the expectant parents to consider what difference an abnormal result would make to their wish to continue with the pregnancy.

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